Permeability tuned loop antenna circuit



June -13, 1950 w. s. BACHMAN PERMEABILITY TUNED LOOP ANTENNA cmcurr Filed Jan. 19, 1945 0 nu 2 m m a o ..m w J n 0 T .b O um W5 A 2 M; M

1% m u K 0 om m m Patented June 13, 1950 PERMEABILITY TUNED LOOP ANTENNA CIRCUIT William S. Bachman, Fairfield, Conn., assignor to General Electric Company, a corporation of New York Application January 19, 1945, Serial No. 573,896

2 Claims.

to provide a new and improved loop antenna circuit for a radio receiver which employs permeability tuning.

It is a further object of the present invention to provide a new and improved permeability tuned loop antenna circuit for a radio receiver which provides both a desirable tuning characteristic over a range of frequencies and a relatively high gain over that range. It is a still further object of the present invention to provide a new and improved low impedance loop antenna circuit employing permeability tuning which has a small amount of capacitive coupling with the electrodes of electron discharge devices employed in the circuit.

It is a still further object of the present invention to provide a new and improved permeability tuned loop antenna circuit which employs a transformer in which the overall coupling coeflicient is maintained substantially constant throughout the tuning range.

It is a still further object of my invention to provide a new and improved loop antenna circuit for a radio receiver in which the necessity for shielding the antenna from the radio frequency circuits of the receiver is avoided.

The features of the invention desired to be protected herein are pointed out in the appended claims. The invention itself, together with further objects andadvantages, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a portion of a radio frequency circuit employing a permeability tuned loop antenna; Fig. 2 is a curve which shows the relation between the gain factor and the coupling between the primary and the secondary windings of the transformer used in the circuit of Fig. 1; Fig. 3 is a cross-sectional view of a transformer which may be employed in the circuit of Fig. 1; Fig. 4 is a modification of the transformer suitable for use in the circuit of Fig. 1; Fig. 5 is a. curve showing the gain and tuning characteristic of the circuit of Fig. 1.

Referring to Fig. 1 of the drawing the portion of a radio frequency circuit there shown includes a loop antenna I which is connected in series with the primary winding 2 of a permeability tuned transformer 3. The transformer 3 includes a secondary winding 4 which is tightly coupled to the primary winding 2 and which is tuned by means of a movable powdered iron core 5. The resonant circuit of the secondary winding 4 includes a capacitance 6 and the output terminal of the circuit is shown as connected to the control electrode of an electron discharge device I.

In the construction of the circuit shown in Fig. 1 I have found that certain limitations on the circuit elements employed are desirable. In particular, it is desirable that both the loop antenna I be constituted by a few turns of wire and the primary winding 2 of the transformer comprise a small number of turns so that the capacitive coupling between the primary and secondary circuits is kept at a small value. It has been observed further that optimum results are obtained by closely coupling the primary winding 2 to the secondary winding 4 and by keeping the primary inductance substantially constant over the entire tuning range of the circuit so that the tuning range of the cou led circuits issubstantially the same as that of the secondary circuit alone.

Furthermore, by so balancing the distribution of the primary circuit inductance between the loop antenna l and the primary winding 2, a maximum gain factor for the overall circuit may be obtained. The desired relation between the inductance of the antenna I and the primary winding 2 may be obtained from a consideration of the curves of Fig. 2 which show the variation of the gain factor L1=L0+Lp, i. e., the total inductance of the primary circuit, Lo=inductance of loop I, L =inductance of primary winding 2, L2=inductance of secondary winding Xc=reactance of apacitor 6, Xto=reactance of loop I, Y

=mutual inductance of transformer 3 Qz=quality of secondary circuit.

In general, the maximum realizable coupling K1 between the primary winding 2 and the secondary winding 4 should be employed, where M K l V P Z The required value of L2 is found from the range of frequencies to be covered, the value of capacitive reactance in the secondary circuit, and theoverall coupling coefficient, 1. e.,

X c XLF 1-10 In arriving at this value of Lz, it has been found,

that the quality Q1 of the primary circuit has substantially small effect on the value of K, the overall coupling coefficient, to be used.

Starting with a suitable value for La, which: for a low impedance input circuit is a small inductance, for maximum gainin the antenna cirwit the totalinductanceLi is distributed between the inductance Lo of, loop I and the in,- ductance Lp of rimary winding. 2 by making L of thesame order and slightly'less than Lo. The

optimum ratioOfwthese component inductances is obtained from the equation In, order to maintain the gain at a maximum substantially constant over the entire tuning 40 range of the circuit. In this transformer struc; ture, the secondary winding 4 is wound as a. solenoid on a dielectric form 8 and the primary winding. 2 is woundon one end of the tuning core 9. formed'ofcompressed powdered iron particles.

The core 9 at its, left-hand end has a shoulder portion-I0 upon. which theprirnary winding 2 is wound. It has been found in the use of-H-a transformer of this constructionthat, as thetuning of, thesecondary circuit is varied-over itscompletecrange, the inductance of the primary circuit is maintained substantially constant'and that-the coupling coefficient of the primary circuit is likewise maintained substantially constant.

Fig. 4 illustrates another form of the transformer construction which is suitable for use in the ,circuit of Fig. 1. In this transformer arrangement, the secondary winding 4 a closely wound solenoid supported on a dielectric form H and the primary windin 2 is wound adjacent to the secondary winding on the form I I at the end into which the tuning core I2 enters. It has been found that in the useofthis particular form of transformer the inductance of. the primary circuit varies by about 2: 1, depending upon the position of the core 2,, and the tuning range of the circuit is extended :by allowing the core, lj2gtopass out of the primary coil 2 and lie whollyc withinthe secondary coil 4.

Fig. 5 illustrates the .gain, and tunin characteristics of the circuit, of Fig. 1 employing the transformer construction shown in Fig. [In the circuit having the characteristics indicated by the curves orrig. 5, the inductanceofwind ing 4 with the tuning core completely removed was approximately 142.4 microhenries, the inductance of the winding 2 was one microhenry, the inductance of the loop antenna I was 2 microhenries, and the capacitor 6 had a value of 58 mmfd. In Fig. 5 curve [3 shows that the gain characteristic of the circuit as its tuned frequency is varied over the normal broadcast range by displacement of the position of the core 9 within the winding 4 is at all points quite high and has an overall variation of not more than 2:1. Curve I4 shows the displacement of the core with frequency and illustrates the tuning characteristic of the circuit.

While the invention has been described by reference to aparticular embodiment thereof, it will be understood that numerous modifications may be made by those skilled in the art without departing, from the invention. I therefore aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of myinvention.

WhatI claim as new and desire to secure by Letters Patent of the United States is:

.1.. In aradio receiver, the combination of a loop antenna, a transformer having, a primary winding serially connected with said antenna, said transformer comprising a tubular dielectric member, said primary Winding being supported on said member, ,a secondary winding supported on said member closely adjacent said primary winding and closely coupled therewith magnetically and so. arranged that the capacitive coupling therewith is minimized, and a magnetic core movable within said member and magnetically coupled to both of said windings for varying the inductance of said secondary winding while maintaining the. inductance ofsaid primary winding substantially constant, said primary winding having an inductance of the same order of but less than the inductance of said loop antenna.

2. In a. radio receiver, the combination of a loop antenna, a transformer having a primary circuit comprising a .primary winding serially connected withsaid antenna, said transformer having a secondary winding closely coupled to saidu primary winding, the inductance of said primary winding totheinductance of said primary, circuitbeing proportional. to

where K:and Kisare thecoupling coefficients of said secondary winding to said: primary circuit and said primary windingxrespectively, and means comprising a. movable core member magnetically coupled toboth of said'windings for varying the inductance ofsaid-secondary winding while maintaining' the inductance of saidprimary circuit and said primary winding substantially constant. BACHMAN REFERENCES. .CLTED following;references-are of -record in the fileiiof. this patent:

UNZTED STATES" PATENTS Number N ame; Date 1,854,915 White ..Apr .::19,1,193'2 2,141,313 Schaper .,Mar; 15,1938 2,267,047. Schaper Dec..23',- 1941 2,283,926 Harveyug May-26,. 1942 OTHER REFERENCES The Aerial: ,Connection The Wireless World. pp. 579-581. 

